Noise limiting circuits



April 2s, 1942. @TR/ws 2,281,395

NOISE LIMITING CIRCUITS Filed July 27, 1939 Patented Apr. 28, 1942 NOISE LEIITING CIRCUITS Charles Travis, Wyndmoor, Pa., assigner, by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application July 27, 1939, Serial No. 286,903 In Great Britain August 26, 1938 (Cl. Z50- 20) 20 Claims.

This invention relates to noise limiting circuits and has for its general object to provide improved means for limiting the amplitude of interfering noise pulses or signals in communication or control systems and the like.

In systems employed in the transmission of television signals, speech, music, and the like, it has been noted that the amplitudes of the Various component frequencies usually vary approximatelyy as an inverse function of frequency. Thus normally only the lower frequencies attain the maximum signal amplitude, while the higher frequenciesmay be of relatively low amplitude.

An ideal noise limiting circuit for such systems would be one which would limit noise pulses accompanying low frequency signals to a level comparable to that of the maximum low frequency signal amplitude, and which would limit noises accompanying higher frequency signals to a lower level comparable to the maximum level of the higher frequency signals. The present invention provides such a system.

According to one feature of the invention, there is provided in a noise limiting circuit, a source of signals and a utilization means therefor, signal transfer means therebetween including a serially connected non-linear impedance means, means for supplying said non-linear impedance means with a biasing or polarizing current whereby said transfer means is rendered substantially non-conducting for signals exceeding predetermined cut-olf amplitudes, and frequency selective circuits associated with said non-linear impedance means whereby said cutoff amplitudes are a function of the sign-al frequency.

According to another feature of the invention there is provided in a noise limiting circuit, a source of signals and a utilization means therefor, signal transfer means therebetween'including a pair of serially connected but mutually reversed non-linear impedance means, means for supplying said non-linear impedance means with a biasing or polarizing current whereby said transfer means is rendered substantially nonconducting for signals exceeding predetermined cut-off amplitudes, and frequency selective circuits associated with said non-linear impedance means whereby said cut-olf amplitudes are a function of the signal frequency.

The invention may be understood by reference to the accompanying drawing, wherein- Fig. 1 is representative of one form of the invention;

Fig. 2 is representative of a preferred embodiment; and

Fig. 3 illustrates a possible cut-off amplitude characteristic as a function of frequency.

Referring generally to Figs. 1 and 2, E represents a source of signals comprising a certain band of frequencies, such as the audio frequencies normally employed in radio reception or the like. A signal utilization means 3 is coupled tothe signal source E by way of a signal transfer means which may include one or a plurality of serially connected non-linear impedance means such as diodes D1 and D2, together with certain associated circuit elements having frequency discriminatory characteristics, such as the resistorcapacitor combinations 5--1 and R10. Where a pair of diodes is employed, the diodes are preferably serially connected with respect to the signal transfer path, butare mutually reversed, as illustrated in Figs. 1 and 2. The signal E may be derived from the usual second detector circuits of a superheterodyne radio receiver from an audio frequency amplifier, or from any other suitable source. The signal utilization means 3 may comprise the usual audio amplifier and loudspeaker, as are well known in the art.

Attention is now directed more specically to Fig. 1 where, in accordance with one embodiment of the invention, a substantially constant current, Ic, from a suitable source S is introduced to the diodes D1 and D2 at their juncture whereby both diodes are maintained in a conductive state for all signals which do not reduce the current through the one or the other diode to zero. The resistors 6 and 8 are provided mainly to complete the direct current paths for the constant current Ic which is divided between the two diodes as indicated. It should be observed that with respect to Ic, which is derived from the source S, the diodes D1 and D2 are effectively in parallel, while with respect to the signal transfer path which extends between the signal source E and the signal utilization means 3, they are in series and mutually reversed.

Assume, by way of example, that the source S supplies a substantially constant current of 2 milliamperes. Under rio-signal conditions this current will divide equally between the two diodes, 1 ma. proceeding to the left through the diode D1 and 1 ma. proceeding to the right through Dz. If now the signal source E supplies an alterhating current having a peak value of one-half ma. to the signal utilization means 3, the current in the diodes will` vary between limits of 0.5 and 1.5 ma., but because the biasing current ows through the diodes in opposite directions with respect to the signal path from E to the device 3 (i. e. from cathode to anode in one and from anode to cathode in the other), the individual diode currents will vary oppositely with respect to one another. Thuswhen the current in D1 is 0.5 ma. the current in Dz will be 1.5 ma., and conversely. Hence the sum of the diode currents will be substantially constant at 2.0 ma., which is the value of current supplied by the constant current source S. If the signal source E supplies an alternating current having a peak value of 1 ma. the current in the diodes will vary between limits of and 2 ma., but since the absolute values of diode current vary in opposite sense the total diode current will again remain substantially constant at 2 ma., the current supplied by the constant current source S. Where,

due to noise conditions or the like, the peak signal supplied by the source momentarily exceeds the limiting value of 1 ma., the current in the two diodes will momentarily assume the limiting values of 0 and 2 ma. respectively, the current supplied by the source S remaining substantially constant at 2 ma.

It will now be understood that when the amplitude of the input signal E exceeds a certain threshold value, the instantaneous current in one or the other of the diodes will be reduced to zero, and accordingly the diode path Di--Dz will be broken and incapable of transmitting signals eX- ceeding such a threshold level. Assuming a symmetrical diode circuit, this limiting threshold level may be made to appear symmetrically on both sides of the zero signal axis, and hence both the positive and negative halves of the signal cycles may be similarly limited. By suitably choosing the value of the constant current Ic derived from the source S, the threshold or cut-off level of the diodes may be made approximately equal to the maximum desired signal amplitude. Noise signals exceeding this level will accordingly be limited to this predetermined cut-off level by the open-circuiting action of the diode path.

It has been found, however, that the amplitudes of the desired audio frequency components which comprise the usual broadcast programs of speech, music, or the like, usually vary according to some inverse function of frequency whereby only the lower audio frequencies attain the maximum signal amplitudes. Accordingly it is an important feature of this invention to provide means whereby the cut-off level of a noise limiter may be made to vary with frequency in such a way as to take advantage of the above fact. In Fig. 1 this desirable feature is attained by associating with the diodes D1 and D2 a suitable frequency selective or discriminatory network which may comprise, for example, a resistor 5 and a capacitor l, The effect of this network is to increase the signal current passing through the diodes at the higher audio frequencies, thus effectively lowering the cut-off level of the transfer path at these frequencies. At the lower frequencies, where the impedance of the network 5-1 is high compared to that of the resistor B the variation of cut-off level with frequency will of course be small. Similarly, at the highest frequencies, where the impedance of the capacitor 'l is very small compared to that cut-off level with frequency will likewise be small.

Such a characteristic of cut-off level vs. frequency is inustrated in Fig. 3. This gure represents the general characteristics of a circuit such as that shown in Fig. 1. By making the direct of the resistorS, the variation of current paths oi the two diodes dissimilar, as by making the resistance of resistor E greater or less than that of the resistor 8, a cut-off characteristic lcan be secured which is unsymmetrical about the zero axis. Moreover, if amplitude limitation is not required for a particular polarity of signal, one of the diodes may be removed from the circuit and replaced by a simple conductor, thu-s giving amplitude limitation for only the negative, or the positive, alternations, as the case may be. Accordingly, the invention is not to be limited to the use of two diodes, since in some applications it may be found that noise pulses extend largely in one direction from the zero axis, in which event a single diode canv provide all desired amplitude limitation.

In Fig. 1, the constant current source S has been indicated only in a general way, there being a number of ways in which such a constant cura rent source may be secured. If desired, the source S may comprise a high voltage battery in series with a resistance which is high compared to all other resistances in the .direct current diode circuits.

It may be observed that the circuit of Fis. 1 will tend to discriminate against the higher signal frequenciesdue to the network which comprises the resistors 5 and 8 and the capacitor 1, which is similar to networks employed in basscompensating tone controls and the like. This discrimination against the higher desired signal frequencies is notan inherent characteristic of the present invention however, and if it is desired to avoid this effect, a constant resistance network may be employed similar to that shown in Fig. 2. The constant resistance network employs the resistors R and R1, the inductor L and the capacitor C. If the resistors R and R1 are ofequal resistance R, and if (R)2 be made equal to L/C, the resistance looking into the network will be constant for all frequencies, and hence the network will not discriminate unequally against component signal frequencies having amplitudes below the cut-off level of the system. In this circuit, as the signal frequency increases, the current through the shunt path R1C increases and hence also the signal current through the diodes, while the current through the shunt path RL decreases. Accordingly, the diodes will limit signals of higher frequencies to a lower relative amplitude level than that at which the lower signal frequencies are limited, and thus the cut-off char# acteristic of Fig. 3 is likewise generally descriptive of the operation of the circuit of Fig. 2.

In the circuit of Fig. 2 there is shown a preferred form of constant current sourcek cornprising a battery 9 and a pentode V1 with cathode load lll, whose rst grid is returned to a point on the cathode load remote from the cathode. The resistors Il and I2 provide constant current paths for the diodes D1 and D2 respectively. It has been found that such a pentode circuit provides a highly constant source of 4direct current, Adue' firstly to its inherently high plate impedance, and secondly due to the degenerative action which effectively tends very substantially to increase the apparent resistance in the pentode circuit by a factor which is a direct function of the amplification factor'of the tube.

It should be obvious, of course, that in either of the circuits shown it will be possible to secure almost any characteristic of cut-off amplitude vs. frequency that might be desired merely by providing the proper type of'frequency discriminatory or selective network. Thus in Fig.2 acharacteristic might be obtained which is just the reverse of that shown in Fig. 3 merely by interchanging the capacitor C and the inductor L.

In the foregoing, it has been assumed that Ie is held constant. However, it may be desirable in some instances to have I@ vary in accordance with certain conditions. Thus, if E be taken as the output of the second detector in a superheterodyne radio receiver, it might be well to have Ic vary as some direct function of the detector output level. In this way, the limiter could be made to operate effectively under a wide range of conditions, regardless of variations in signal strength and the like. For example, in Fig. 2 the short-circuiting conductor I3 might be removed from the terminals I4, and a voltage source l5 connected thereacross Whose voltage varies directly with the average amplitude of the modulated intermediate frequency carrier Wave, as is Well known in the automatic volume control art.

Although the invention has been described with particular reference to the embodiments of the drawing, it will be understood that the invention is capable of various forms of physical expression, and is not to be limited to the specic disclosure, but only by the scope of the appended claims.

I claim:

1. A noise limiting circuit comprising a source of signals and a utilization means therefor, signal transfer means therebetween including a serially connected non-linear impedance means, means for supplying said non-linear impedance means with` a biasing current, .'whereby said transfer means is rendered substantially nonconducting for signals exceeding predetermined cut-off amplitudes, and frequency selective circuits operatively associated with said non-linear impedance means for varying said cut-off amplitudes according to a function of the signal frequency.

2. A noise limiting circuit comprising a source of signals and a utilization means therefor, signal transfer means therebetween including a pair of serially connected but mutually reversed non-linear impedance means, means for supplying said non-linear impedance means with a biasing current, whereby said transfer means is rendered substantially non-conducting for signals exceeding predetermined cut-off amplitudes, and frequency selective circuits operatively associated with said non-linear impedance means for varying said cut-oil? amplitudes according to a function of the signal frequency.

3. A noise limiting circuit in accordance with claim 2, characterized in that the non-linear impedance means comprises a pair of mutually reversed vacuum tube diodes.

4. A noise limiting circuit having high and low potential input and output terminals, connections between said low potential input and output terminals, a pair 0f serially connected, mutually reversed diodes connected between said high potential input and output terminals, a voltage source for supplying said diodes with a biasing current, said biasing voltage source being connected between the low potential side of said circuit and the junction between said diodes, whereby the diode path is rendered substantially non-conducting for signals exceeding predetermined cut-off amplitudes, and a frequency selective circuit associated with said diodes whereby said cut-off amplitudes are caused to vary with the signal frequency.

CII

5..A noise limiting'circuit in accordance with claim 4', wherein said frequency selective circuit comprises a resistor and a capacitor connected in series and shunted across said output terminals.

6. A noise limiting circuit having high and low potential input and output terminals, connections between said low potential input and outputterminals, a pair of serially connected, mu-

tually reversed diodes connected between said high potential input and output terminals, a voltage source for supplying said diodes with a biasing current, said biasing voltage source being connected between the low potential side of said circuit and the junction between said diodes, whereby the diode path is rendered substantially non-conducting for signals exceeding predetermined'cut-off amplitudes, means for maintaining said biasing current substantially constant, and a frequency selective circuit associated with said diodes whereby said cut-off amplitudes are caused to vary with the signal frequency.

7. A noise limiting circuit having high and low potential input and output terminals, connections between said low potential input and output terminals, a pair of serially connected, mutually reversed diodes connected between said high potential input and output terminals, a voltage' source for supplying said diodes with a biasing current, said biasing voltage source being connected between the low potential side of said circuit and the junction between said diodes, whereby the diode path is rendered substantially non-conducting for signals exceeding predetermined cut-of amplitudes, means for varying the magnitude of said biasing current in accordance with the average amplitude of the modulated carrier wave, and a frequency selective circuit associated with said diodes, whereby said cutoff amplitudes are caused to vary with the signal frequency.

8. A noise limiting circuit comprising a source of signals and a utilization means therefor, signal transfer means therebetween including a serially connected non-linear impedance means, means for supplying said non-linear impedance means with a biasing current, whereby` said transfer means is rendered substantially non-conducting for signals exceeding predetermined cut-off amplitudes, said biasing means comprising a source of unidirectional current in series with the plate circuit of a degeneratively connected vacuum tube, and frequency selective circuits associated with said non-linear impedance means, whereby said cut-oi amplitudes are a function of the signal frequency.

9. A noise limiting circuit having high and low potential input and output terminals, connections between said low potential input and output terminals, a pair of serially connected, mutually reversed diodes connected between said high potential input and output terminals, a voltage source connected between the low potential side of said circuit and the junction between said diodes for supplying said diodes with a. biasing current, whereby the diode path is rendered substantially non-conducting for signals exceeding predetermined cut-olf amplitudes, said bias voltage source comprising a source of unidirectional current in series with the plate circuit of a pentode having a cathode resistor, the control grid of said pentode being returned to a point on said cathode resistor, and a frequency selective circuit associated with said diodes, whereby said cut-off amplitudes are caused to vary with the signal frequency.

10. A noise limiting circuit in accordance with claim 9, characterized in that a source of runidirectional voltage is inserted between said grid and said cathode resistor, there being means provided to vary the magnitude of said voltage in accordance with the amplitude of the desired signal.

1l. A noise limiting circuit having high and low potential input and output terminals, connections between said low potential input and output termina1s,.a pair of serially connected, mutually reversed diodes connected between said high potential input and output terminals, a voltage source for supplying said diodes with a biasing current, said biasing voltage source being connected between the low potential side of said circuit and the junction between said diodes, whereby the diode path is rendered substantially non-conducting for signals exceeding predetermined cut-off amplitudeaa series frequency selective circuit comprising a resistor and a ca pacitor connected across said output terminals, anda second series frequency selective circuit comprising a resistor and an inductor connected across said input terminals, said resistors, capacitor and inductor being so related that said limiting circuit presents a substantially constant resistance over a wide band of frequencies, while simultaneously having a cut-off level which varies inversely with frequency.

12. A noise limiting circuit in accordance with claim 11, including means independent of at least one of said frequency selective circuits for completing the direct current circuit for said biasing current.

13. In a noise limiting circuit, a constant current source, a pair of diodes, a first impedance for connecting one of said diodes in shunt with said constant current source, a second impedance for connecting the other o'f said diodes in shunt with said constant current source, a source of signals connected to one of said impedances, and a signal utilization means connected to the other of said impedances.

14. In an amplitude limiting circuit, a vacuum tube having at least a cathode, a grid, and an anode, an impedance having one end connected to said cathode, a source of substantially constant voltage having one terminal thereof connected to the other end of said impedance, a connection between said grid and a point on said impedance, said vacuum tube, impedance, and voltage source constituting a source of substantially constant current, a pair of diodes, an impedance connecting one of said diodes in shunt with said constant current source, another impedance connecting the other of said diodes in shunt with said constant current source, a source of signals connected to one of said last-mentioned impedances. and a signal utilization means connected to the other of the said impedances.

i5. In an amplitude limiting circuit, a vacuum tube having at least a cathode, a grid, and an anode, an impedance having one end connected to said cathode, a source of substantially ccnstant voltage having one terminal thereof connected to the other end of said impedance, a connection between said grid and a point on said impedance, said vacuum tube, impedance, and voltage source constituting a source of substantially constant current, a pairr of diodes, an im pedance connecting one of said diodes in shunt with said constant current source, another impedance connecting the other of said diodes in shunt with said constant current source, a source of signals connected to one of said last-men ticned impedances, a signal utilization means connected to the other of the said impedances, and a frequency selective network associatedv with at least one of said diodes, whereby the cut-on amplitude of said circuit is caused to vary in accordance with the frequency of the signals derived from said source.

16. A noise limiting circuit, comprising a source of signals and a utilization means therefor, signal transfer means therebetween inc1uding a serially connected non-linear impedance, means for passing a biasing current through said non-linear impedance, whereby said trans fer means is rendered substantially ncn--conn` ducting for signals exceeding a predetermined cut-off amplitude, said biasing current being in` terrupted only in the presence of noise signals exceeding said cut-off amplitude, and frequency selective circuits operatively associated with said non-linear impedance for varying said cut-off amplitude as a function of the signal frequency.

17. A noise limiting circuit, comprising a source of signals and a signal utilization means therefor, signal transfer means therebetween including a device for limiting the amplitude of transferred signals to a ypredetermined cut-ofi amplitude, the gain and band width of said sig nal transfer means being substantially unaffected by said amplitude limiting device, and frequency responsive means operatively associated with said device for varying said cut-off amplitude in accordance with a predetermined function of the signal frequency.

18. A noise limiting circuit, comprising a source of signals and a signal utilization means therefor, signal transfer means therebetween in cluding a device for limiting the amplitude of transferred signals to a predetermined cut-off amplitude, the gain and band width of said signal transfer means being substantially unaffected by said amplitude limiting device, and frequency responsive means operatively associated with said device for varying said cut-off amplitude in accordance with an inversefunction of the signal frequency.

19. A noise limiting circuit according to claim 18, characterized in the provision of means for limiting both the positive and negative alternations of the signal to substantially the same amplitudes.

20. In a signaling system, a signal channel adapted to transfer desired signals, anamplitilde-limiting device in said channel, means for rendering said device conductive only to signals whose amplitude does not exceed a predetermined cut-off amplitude, whereby said device becomes substantially non-conducting for sige nals whose amplitude exceeds said cut-off amplitude, and frequency responsive means operatively associated with said device for varying said cut-off amplitude as a function of the signal frequency. y

CHARLES TRAVIS. 

